What is the difference between:
HTML
SHTML
PHTML
Why and when i'll use which one?
Use static HTML when you want to have static content. Use SHTML if you want to do simple things such as includes. Use PHTML if you want to use logic (though it's possible to do with SHTML if you really want to).
.phtml is usual extension to Phalcom PHP Framework files :)
Ref.: https://docs.phalconphp.com/en/latest/reference/tutorial.html
It looks like SHTML = Server Side include HTML (SSI), which is just a fancy way of saying the server can dynamically inject code at the reference point (i.e. where the include is).
PHTML is HTML code with inline PHP processing instructions.